Spatial Synergy of Crown Ether Groups in Covalent Organic Framework Nanochannels Enabling Efficient Monovalent Cation Separation.

Selective ion transport in nanochannels is critical not only for ion extraction but also for analytical applications requiring high-precision ion isolation prior to detection. However, for ions with identical charge and similar size (e.g., Li+, Na+, K+), it is usually difficult to create sufficiently large differences in transport behavior to achieve high selectivity. Here, we report a covalent organic framework (COF) membrane featuring spatially organized 18-crown-6 sites. These uncharged "-CH2-O-CH2-" groups leverage weak yet size-specific coordination with alkali metal ions. The size compatibility and stable binding of K+ with 18-crown-6 significantly lower its transport energy barrier and enhance its flux, yielding monovalent cation selectivity up to 14.5. Furthermore, we demonstrate that K+ ion can be rapidly delivered between the rotatable neighboring 18-crown-6 sites in the COF nanochannel, following a "rotate-and-deliver" mechanism. This study sheds lights on the fundamentals of ion transport within subnanoconfined systems and would stimulate the design of high-performance membranes.